The use of fiber optics for transmitting information has recently received a great deal of attention because of the light weight, security, safety, and electrical isolation that can be obtained with a fiber optic system, and the enormous amount of information that can be transmitted through each fiber of a fiber optic system. Fiber optic systems use a waveguide for transmitting light between a light emitter and a light detector. Waveguides consisting of a variety of materials have been developed. For example, waveguides consisting of a glass core and glass cladding, glass cladding and a liquid core, a polymeric core and polymeric cladding, and a glass core and polymeric cladding are known. U.S. patent application Ser. No. 964,506 filed by Ellis et al on Nov. 29, 1978, now U.S. Pat. No. 4,290,668, which is incorporated herein by reference, is directed to waveguides comprised of a quartz glass core and polymeric cladding of polydimethyl siloxane.
An important property of a waveguide is its numerical aperture. Light energy entering the end surface of a waveguide is accepted and transmitted down the core only for those entry angles within an acceptance cone. The half angle of the "acceptance cone" or "numerical aperture" (NA) is a function of the core/cladding indices of refraction as follows: EQU NA=Sin .theta..sub.c =(n.sub.1.sup.2 -n.sub.2.sup.2).sup.1/2
where
n.sub.1 =index of refraction of the core; PA1 n.sub.2 =index of refraction of the cladding; and PA1 .theta..sub.c =acceptance cone half-angle. PA1 (i) alkyl groups containing from one to eight carbon atoms; PA1 (ii) fluorine substituted alkyl groups containing from 3 to 8 carbon atoms with no fluorine atoms at the alpha and beta carbon atoms; and PA1 (iii) polyfluoroalkoxy alkyl groups containing up to 8 carbon atoms and at least 2 fluorine atoms with the alkoxy group having from 1 to 6 carbon atoms and the alkyl group having from 2 to 6 carbon atoms with no fluorine substitution of the alpha and beta carbon atoms of the alkyl groups, where a sufficient portion of the R.sub.1 and R.sub.2 groups is a fluorine substituted group so that the polyfluorosiloxane contains at least 1% by weight fluorine. The repeating units can be the same or different.
Thus, the numerical aperture of a waveguide is proportional to the difference between the refractive index of the core and the refractive index of the cladding. The higher the numerical aperture of a waveguide, the greater the percentage of light provided by a light emitter that enters the core of the waveguide. Therefore, it is advantageous for efficient transmission of light to use a waveguide with a high numerical aperture.
Another advantage of using a waveguide with a high numerical aperture is that alignment between a light emitter and a waveguide is less critical. This permits smaller cores to be used in a waveguide, and permits workers in the field to partially misalign a waveguide and a light emitter, without having intolerable light losses.
A problem with a waveguide made with polydimethyl siloxane cladding and a quartz glass core is that polydimethyl siloxane has a refractive index of approximately 1.40, while the quartz core has a refractive index of about 1.46. Thus, the numerical aperture of this combination of materials is only about 0.414. It would be desirable to have claddings that have a refractive index lower than 1.40 so that wave guides with numerical apertures higher than about 0.45 can be produced.
There are other problems with use of polydimethyl siloxane as a cladding. For example, it has limited use at low temperatures. A frequent military requirement for communication systems is operability at temperatures lower than -55.degree. C. Polydimethyl siloxane can crystallize at about -50.degree. C., which can give rise to an increase in optical attenuation. Thus, silicone cladding generally is not used for very low temperature applications.
Another problem with many types of polymeric cladding materials is relatively poor oil resistance. In some applications, such as systems proximate to hydraulic mechanisms, it is important that the cladding be oil resistant. If it were not, the cladding could absorb some oil, which could change the light transmitting properties of the waveguide.
In view of the foregoing, it is apparent that there is a need for a waveguide cladding that is oil resistant, has a low refractive index, and which can be used at temperatures lower than -55.degree. C. without adversely affecting the operational and mechanical properties of the waveguide.